I switch brands (all high quality) every bag, which works out to be every month or 6 weeks for my guys. All of them are now at the point that we can switch cold turkey no problem and that does certainly help if you run out of dog food lol.
That being said, I've heard of several people trying to switch every meal and that seems to be too much for the dogs. As for mixing, I'd probably just stick with one food at a time so that if there ever was something making your pup sick you could narrow down the food quickly.